Configurable Buffer For An Integrated Circuit

ABSTRACT

In one embodiment, an internal buffer may be provided within an integrated circuit (IC) to convert a signal to an output current to be output via a pin of the IC, under control of a switch which can be controlled based on a configuration setting of the IC, and may selectively directly couple the signal to the pin when the IC is coupled to an external driver circuit.

BACKGROUND

Certain integrated circuits (ICs) can be configured in many differenttypes of systems. Often times, an output from the IC can be coupled toother circuits in a system via some kind of interim or intermediatecircuitry. This circuitry can take different forms in various systemssuch as AC-coupled circuits and DC-coupled circuits. To enableinterfacing of an IC that has a specific type of output, oftentimes thisinterim circuitry can be provided by way of an AC-coupled externaldriver to thus receive the IC output and buffer it into an appropriateform for coupling to another location.

While these buffer circuits typically work well, they increase the size,cost and complexity of a system. Furthermore, the IC has limitedflexibility to be incorporated into various devices, as the IC isgenerally configured to provide only a particular type of output signal(i.e., AC or DC coupled outputs).

SUMMARY OF THE INVENTION

According to one embodiment, an apparatus includes an amplifierconfigured on an integrated circuit (IC) to receive and amplify an inputsignal received via a first input terminal of the amplifier to anamplified signal to be output via a first pin of the IC. In turn, afeedback loop may be coupled between an output of the amplifier and thefirst input terminal. This loop may include a first path having a switchenabled to selectively directly couple the amplified signal to the pinand a second path having a buffer to convert the amplified signal to anoutput current to be output via the pin, when the switch is disabled.The switch can be controlled based on a configuration setting of the IC,and may selectively directly couple the amplified signal to the pin whenthe IC is to couple to an external driver circuit, and be disabled whenthe IC is to directly couple to a predetermined impedance.

In an embodiment, the buffer may include a converter to convert theamplified signal to a current signal, a first current mirror to receivethe current signal and to amplify the current signal to a firstamplified current signal, and a second current mirror coupled to thefirst current mirror to receive and amplify the first amplified currentsignal to an output current, and to output the output current to thepin.

Another aspect of the present invention is directed to a methodincluding reading a configuration register of a tuner to obtain acontrol value, controlling switches based on the control value,receiving and processing a radio frequency (RF) signal in the tuner tooutput a driven CVBS signal from a drive circuit of the tuner, andoutputting the driven CVBS signal directly from the driver circuit to anoutput pin of the tuner when the tuner is coupled to an AC-coupled load.The driven signal can be buffered in an internal buffer of the tuner andcoupled to the output pin when the tuner is coupled to a DC-coupledload.

Yet another aspect of the present invention is directed to a system thatincludes an antenna to receive an RF signal and a tuner coupled to theantenna. In one implementation, the tuner includes a mixer to mix the RFsignal to a second frequency signal, a digitizer to digitize the secondfrequency signal, a digital signal processor (DSP) coupled to thedigitizer to process the digitized second frequency signal and todemodulate the processed digitized second frequency signal when the RFsignal includes an analog TV signal, a driver to output the analog TVsignal to an output pin in a first mode, and a buffer coupled to thedriver to receive the analog TV signal and to provide the analog TVsignal to the output pin as a buffered signal in a second mode. Thesystem may include a configuration register to store a control valueused to enable the buffer in the second mode when the tuner is coupledto a processor via a transmission line and to disable the buffer in thefirst mode when the tuner is coupled to the processor via an externaldriver.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a circuit in accordance with anembodiment of the present invention.

FIG. 2 is a schematic diagram of a transmission line driven by anexternal driver.

FIG. 3 is a schematic diagram of a transmission line to which a tuner inaccordance with an embodiment of the present invention may be coupled.

FIG. 4 is a block diagram of a tuner in accordance with an embodiment ofthe present invention.

FIG. 5 is a flow diagram of a method of operating a circuit inaccordance with an embodiment of the present invention.

FIG. 6 is a block diagram of a system in accordance with one embodimentof the present invention.

DETAILED DESCRIPTION

In various embodiments, an integrated circuit (IC) can be provided withan internal buffer to provide multiple modes of operation of the circuitand to enable its coupling to various system configurations with greatamounts of flexibility. For purposes of discussion, a representativecircuit including an internal buffer is for a TV tuner that isintegrated on a single semiconductor die, such as a single-diecomplementary metal oxide semiconductor (CMOS) die. However, understandthe scope of the present invention is not limited in this regard, andembodiments of an internal controllable buffer can be implemented inmany other types of ICs, driving (e.g., 75 ohm) transmission lines, suchas radio frequency (RF) or intermediate frequency (IF) circuits.

Referring now to FIG. 1, shown is a schematic diagram of a circuit inaccordance with an embodiment of the present invention. As shown in FIG.1, circuit 10 can be used to provide a configurable option to outputsignals from a tuner in different states depending on what is connectedto the tuner. More specifically circuit 10 can include an internalbuffer 30, which in an embodiment can drive a 75 ohm transmission lineand a 75 ohm DC-coupled resistive load to buffer a voltage signal into acurrent signal so that it can be output to a DC-coupled output such as atransmission line without an external buffer. Alternately, the buffercan be controlled to be disabled such that instead an output voltage isprovided to an external buffer that can be implemented e.g., via anAC-coupled external driver. As will be described further below, invarious embodiments this circuit can be configured via programmablemechanisms via a controller or other logic, e.g., as set by a systemconfiguration which can be represented by a control value stored in aconfiguration register or other location.

As seen in FIG. 1, circuit 10 includes an input (In) to receive anincoming signal which may be provided from a processing circuit to anamplifier 20. In the embodiment shown, amplifier 20 can be implementedas an operational amplifier (op amp). To this end the incoming signalcan be coupled via a voltage divider formed of resistors R2 and R3 to afirst (inverting) input terminal of amplifier 20. In turn, a second(non-inverting) input terminal of amplifier 20 may be provided with abias voltage V_(b). Amplifier 20 acts as a driver to thus receive thisinput voltage and generate an output voltage.

Depending on configuration of circuit 10, this output voltage may beprovided to an output node, Out, which may be coupled to an output pinof an integrated circuit (IC) including the tuner. This configurationcan be realized by enabling a first switch S1 to be closed such that ashort circuit is present from the output terminal of amplifier 20 to thecircuit output node to thus provide an output voltage at this outputterminal. Note that when switch S1 is enabled (i.e., closed) this bypassswitch thus shorts an internal buffer 30 (which may be disabled in thisconfiguration). As this switch is inside a feedback loop of theamplifier, a relatively small switch can be used without significantdegradation of the amplifier. This mode may be enabled when there is anAC-coupled external driver or other such AC-coupled circuit coupled tothe tuner.

In contrast, when the circuit is to be coupled without such externalbuffer, internal buffer 30 may instead be used. To enable this circuit,switch S1 may be opened or turned off, thus enabling buffer 30. As seen,buffer 30 when enabled may have switch S2 closed (i.e., on) and switchesS3 and S4 opened (i.e., off) (and all switches S2-S4 vice versa when thebuffer is not enabled). In general, buffer 30 may include a converterand a pair of amplifiers, namely a dual-stage current amplifier. Theconverter may be a voltage-to-current converter and can be implementedin the embodiment shown via a resistor R1, thus generating an inputcurrent that may be provided to gate terminals of a pair of metal oxidesemiconductor field effect transistors (MOSFETs) M1 and M2, which may bepart of a first current mirror 40 formed of these two NMOS devices. Notethat with switch S2 on MOSFET M1 is in a diode-connected configurationwith commonly coupled gate and drain terminals. Both transistors M1 andM2 have their source terminals coupled to a predetermined referencevoltage node, which in an embodiment can be a ground voltage node.

In turn, the resulting output of this current mirror may be provided viaa drain terminal of MOSFET M2 to an input terminal of a second currentmirror 50. As seen, this current mirror may be formed of a pair ofMOSFETs M3 and M4, more specifically a pair of PMOS devices havingcommonly coupled gate terminals. As seen, the resulting amplified outputcurrent can be provided from a drain terminal of MOSFET M4 as an outputcurrent I_(out) provided to output node Out.

As further shown, a pair of current sources I₀ and I₁ may be present andcan be used to bias current mirror 40. More specifically, the firstcurrent source I₀ may set a gate-to-source voltage of MOSFET M1 to avoltage of approximately VDD/2, providing a balanced load to amplifier20. In turn, second current source I₁ sets a V=0 condition. Note thatthe value of resistor R1, and the ratio of M1, M2, M3, M4 may determinethe relation between the output swing of amplifier 20 and the outputcurrent range. In an embodiment, this ratio may be close to, but lessthan 1. The enabled buffer is inside the feedback loop of the amplifier20. In an embodiment, the ratio may be

${{\frac{1}{R_{1}} \times \frac{M\; 2}{M\; 1} \times \frac{M\; 4}{M\; 3} \times {RL}} < 1},$

where RL is the total load impedance of 150 ohms.

As described above, by incorporation of an internal buffer, such as the75 ohm buffer shown in FIG. 1, tuner 10 can be incorporated intodifferent system implementations, namely an implementation including anAC-coupled external driver, and in a different system instead providinga DC-coupled 75 ohm output.

Referring now to FIG. 2, shown is a schematic diagram of an outputdriver to which a tuner in accordance with an embodiment of the presentinvention may be coupled. As shown in FIG. 2, circuit 60 may be anAC-coupled external driver including a bipolar transistor T1 that isAC-coupled to receive the output of a tuner via an AC-coupling capacitorC1 and a voltage divider presented by resistors R10 and R11. As seen,transistor T1 may be configured as an emitter follower receiving thisAC-coupled signal via its base terminal and outputting the signal to atransmission line via a 75 ohm resistor (R12) and a DC blockingcapacitor (C2) and on to a video decoder 65 including a 75 ohm resistorR13.

Instead in other implementations, a tuner in accordance with anembodiment of the present invention may be directly DC-coupled to aprocessor via a transmission line such that the tuner can directly drivea CVBS line input to a video processor or other such processor via atransmission line. As seen in FIG. 3, circuit 70 may be a DC-coupledarrangement with a pair of termination resistances RESD+R20 and R21coupled at either end of a transmission line 80 that may couple thetuner to a video processor, namely video decoder 85. (The inputtermination of the transmission line is dominated by the sum of R20 andthe electrostatic discharge output impedance (R_(ESD)) of the driver(shown in FIG. 4). Both terminations can match the characteristicimpedance of transmission line 80, which is 75 ohm by standard.) Notethat while shown in FIG. 3 as being implemented as a cable, understandthat the transmission line can take the form of any type of conductiveelement, from a circuit board trace, a cable such as a coaxial cable orany other type of conductive element.

Referring now to FIG. 4, shown is a block diagram of a tuner inaccordance with an embodiment of the present invention. In theembodiment shown, tuner 100 may be configured to receive a variety ofdifferent incoming RF signals. In implementations in which tuner 100 isa television tuner, the incoming RF signal can be one of a variety ofworld wide terrestrial and cable TV standards including both analog anddigital TV-based standards.

As seen in FIG. 4, incoming RF signals may be coupled through an antenna105 to tuner 100 which as generally shown is coupled between an inputpin A and an output pin B. Of course understand while only these twoconnections are shown for ease of illustration and reference to thebuffer circuitry described herein, understand that many different pinsmay be provided on the tuner. Furthermore, the single-ended circuit isshown for ease of illustration, in other embodiments tuner 100 canhandle incoming differential signals via a complex signal processingpath. In many implementations tuner 100 can be configured on a singlesemiconductor die such as a CMOS die. As such, the internal buffer maybe configured solely of CMOS devices, in contrast to conventionalexternal buffers, which are typically formed of bipolar devices.

Incoming signals may be provided to an RF front end unit 110 which maygenerally receive and handle processing of the signals, e.g., viaamplifiers or other gain control circuitry. The resulting signals arecoupled to a mixer 120 which operates to downconvert the RF signals tolower frequency signals. In various embodiments, tuner 100 can beconfigured to be a low-IF tuner such that mixer 120 operates todownconvert the RF signal to a low-IF signal. However, understand thescope of the present invention is not limited in this regard. Mixer 120may downconvert the signal responsive to a local oscillator signalreceived from a phase lock loop (PLL) 115 which in an embodiment can beimplemented via a frequency synthesizer. Although shown as asingle-ended mixer, understand that in various embodiments this mixermay be a complex mixer.

The resulting lower frequency signal is provided to an analog-to-digitalconverter (ADC) 130 which in an embodiment can be a delta-sigma ADC tothus digitize the low-IF signal and provide it to a digital signalprocessor (DSP) 140 which may perform various processing, depending uponthe type of signal. For an analog TV signal, DSP 140 may perform variousoperations including demodulation of the signal to thus provide aresulting demodulated signal. Instead for digital signals, the DSP mayperform other processing to generate a digital low-IF signal that can beprovided to off-chip circuitry, such as via a separate digitaldemodulator path (not shown for ease of illustration in FIG. 4).However, understand that in some implementations it is possible tofurther perform digital demodulation within tuner 100.

Still referring to FIG. 4, after processing, the resulting digitalsignals may be provided to a digital-to-analog converter (DAC) 150 whichthus converts the signal back to an analog signal. For purposes ofillustration, assume the processed signal is a demodulated analog TVsignal. As seen, the signal is provided to a driver 160 which maygenerally correspond to op amp 20 of FIG. 1. After amplification in thedriver, the resulting signal, which may correspond to a CVBS signal, canbe output via a buffer 170 which may be configured the same as buffer 30of FIG. 1 to thus provide a CVBS signal which can be put through atermination resistance R_(ESD) via output pin B. This is so, as switchS1 is opened in the configuration of FIG. 4 because tuner 100 is coupledto a 75 ohm impedance connection that includes a 75 ohm terminationresistance formed of a source resistance R_(S) and R_(ESD) coupled to atransmission line 80 that in turn is coupled to a video decoder 85including a 75 ohm termination resistance. Note that in variousembodiments, attenuation of the CVBS signal may occur due to theseresistances. Of course also understand that in a different configurationof tuner 100, switch S1 can be closed (i.e., active or on) to thusdirectly couple the voltage output of driver 160 to output pin B wheninstead tuner 100 is coupled to an external buffer such as driver 60 ofFIG. 2. Note that the R_(ESD) is always present. When S1 is closed, theoutput is a voltage driver with an output impedance of R_(ESD), whichmay typically be 28 ohm. In this mode the output is not loaded by a 75ohm transmission line, but other circuits (e.g., an external 75 ohmbuffer as in FIG. 2) which load the driver with a much higher impedance,e.g., 1-10 kilo ohms. So in this case the R_(ESD), is negligible. Butwhen the 75 ohm transmission line is directly loading the driver, theR_(ESD) is not negligible and is included in calculating the inputtermination impedance. Although shown at this high level in theembodiment of FIG. 4, understand the scope of the present invention isnot limited in this regard.

Referring now to FIG. 5, shown is a flow diagram of a method ofoperating a circuit in accordance with an embodiment of the presentinvention. As shown in FIG. 5, method 200 can be implemented within anIC, such as a TV tuner that is configurable to disable an internalbuffer, such as the 75 ohm internal buffer of FIG. 1. As seen in FIG. 5,method 200 can begin by reading a configuration register to obtain acontrol value (block 210). In an embodiment, this configuration registercan be set by a system vendor and be stored in a non-volatile storagewhere the value can be loaded into a configuration register on power up.From this configuration register the control value, which may be a setof bits, can be obtained. In one embodiment, each of a plurality of bitscan be used to control a given switch such as switches S1-S4 shown inthe embodiment of FIG. 1.

Accordingly as seen in FIG. 5, at block 220 a plurality of switches canbe controlled based on the control value. More specifically, it can bedetermined whether a load coupled to the tuner is DC-coupled (diamond230). Note that this determination may be based on the configurationregister setting, which as described above can be originally set onimplementation of a tuner into a given system that either has or doesnot have a DC-coupled load to the tuner. When the tuner is implementedin a system having a DC-coupled load, the control of the switches may besuch that the internal buffer is enabled. Thus at block 250 a signaloutput from a driver of the tuner can be delivered to an output pin ofthe tuner via the internal buffer. Instead, when a AC-coupled load ispresent, at block 240 the output signal from the driver may be directlycoupled to the output pin to thus enable an AC-coupled arrangement,e.g., via an AC-coupled external driver. Note that although shown in theembodiment of FIG. 5 as including a determination as to whether aDC-load is coupled, note that a controller does not actually make thisdetermination; instead simply based on the control value, the controlswitches will be arranged accordingly, either to enable or disable theinternal buffer. Note also that a technique to automatically determinethe nature of the load can also be used to identify the load andconfigure the circuit accordingly.

Embodiments may be implemented in many different system types, such aswireless devices, set-top boxes, televisions, and so forth. Someapplications may be implemented in a mixed signal circuit that includesboth analog and digital circuitry. Referring now to FIG. 6, shown is ablock diagram of a system in accordance with one embodiment of thepresent invention. As shown in FIG. 6, system 1000 may include atelevision that is coupled to receive a RF signal from an antenna source1001 such as an over-the-air antenna 1001. However, in other embodimentsthe original source may be cable distribution, satellite, or othersource that is then redistributed through a digital terrestrial network.The incoming RF signal may be provided to a tuner 1010 which may be, inone embodiment, a single-chip mixed signal device including both a tunerand a demodulator for analog signals.

More specifically, the incoming RF signal is provided to an analog frontend 1005 of the tuner for tuning to a desired signal channel. While thescope of the present invention is not limited in this regard, front end1005 may include various circuitry. For example, in one embodiment frontend 1005 may include a bandpass filter having an output coupled to a lownoise amplifier (LNA) to receive and amplify the RF signal. The outputof the LNA may be provided to another bandpass filter that in turn iscoupled to a mixer. In turn, the mixer downconverts the incoming RFsignal to a complex output. This complex output (i.e., I/Q data) may beat IF, low-IF, or zero-IF in different systems. As shown in FIG. 6,analog front end 1005 may be coupled to analog-to-digital converters(ADCs) 1012 to convert the incoming I/Q data to digital form, to providea modulated bitstream of a desired signal channel. The signal channelinformation may be provided to a digital signal processor (DSP) 1015which can perform various digital-based processing operations, includinganalog demodulation to obtain a CVBS signal and a SIF/AF signal that canbe provided to a host processor 1020. Host processor 1020 may furtherprocess the information into an audio visual signal that may be providedto a display 1030, such as a computer monitor, flat panel display orother such display.

While the present invention has been described with respect to a limitednumber of embodiments, those skilled in the art will appreciate numerousmodifications and variations therefrom. It is intended that the appendedclaims cover all such modifications and variations as fall within thetrue spirit and scope of this present invention.

What is claimed is:
 1. An apparatus comprising: an amplifier configuredon an integrated circuit (IC) to receive and amplify an input signalreceived via a first input terminal of the amplifier to an amplifiedsignal to be output via a first pin of the IC; a feedback loop coupledbetween an output of the amplifier and the first input terminal, thefeedback loop including: a first path having a first switch enabled toselectively directly couple the amplified signal to the first pin; and asecond path having a buffer to convert the amplified signal to an outputcurrent to be output via the first pin of the IC, when the first switchis disabled.
 2. The apparatus of claim 1, wherein the first switch iscontrolled based on a configuration setting of the IC.
 3. The apparatusof claim 1, wherein the first switch is to selectively directly couplethe amplified signal to the first pin when the IC is coupled to anexternal driver circuit, and wherein the first switch is disabled whenthe IC is to directly couple to a CVBS interconnect.
 4. The apparatus ofclaim 1, wherein the buffer includes: a converter to convert theamplified signal to a current signal; a first current mirror to receivethe current signal and to amplify the current signal to a firstamplified current signal; and a second current mirror coupled to thefirst current mirror to receive the first amplified current signal andto amplify the first amplified current signal to an output current, andto output the output current to the first pin.
 5. The apparatus of claim4, wherein the first current mirror includes: a first transistor and asecond transistor having commonly coupled gate terminals and commonlycoupled first terminals, and wherein a second terminal of the firsttransistor is coupled to the commonly coupled gate terminals when thebuffer is enabled.
 6. The apparatus of claim 5, wherein a secondterminal of the second transistor is coupled to the second currentmirror.
 7. The apparatus of claim 5, further comprising a second switchto couple the gate terminals of the first and second transistors to thefirst terminals of the first and second transistors when the buffer isdisabled.
 8. The apparatus of claim 5, wherein the second current mirrorincludes: a third transistor and a fourth transistor having commonlycoupled gate terminals and commonly coupled first terminals, and whereina second terminal of the third transistor is coupled to the commonlycoupled gate terminals when the buffer is enabled.
 9. The apparatus ofclaim 8, further comprising a third switch to couple the gate terminalsof the third and fourth transistors to the first terminals of the thirdand fourth transistors when the buffer is disabled.
 10. The apparatus ofclaim 5, further comprising a first current source to provide a firstbias current to the second terminal of the first transistor.
 11. Theapparatus of claim 5, further comprising a second current source toprovide a second bias current to the second terminal of the secondtransistor.
 12. A method comprising: reading a configuration register ofa tuner to obtain a control value, the tuner comprising a single chipcomplementary metal oxide semiconductor (CMOS) tuner; controlling aplurality of switches of the tuner based on the control value; receivingand processing a radio frequency (RF) signal in the tuner to output adriven signal from a drive circuit of the tuner; and outputting thedriven signal directly from the driver circuit to an output pin of thetuner when the tuner is coupled to an AC-coupled load, and otherwisebuffering the driven signal in an internal buffer of the tuner andcoupling the buffered signal to the output pin when the tuner is coupledto a DC-coupled load.
 13. The method of claim 12, further comprisingcontrolling the plurality of switches to disable the internal bufferwhen the tuner is coupled to the AC-coupled load.
 14. The method ofclaim 13, wherein controlling the plurality of switches includes:enabling a first switch to directly couple the driven signal to theoutput pin, disabling a second switch of the internal buffer, andenabling third and fourth switches of the internal buffer.
 15. Themethod of claim 14, wherein controlling the plurality of switchesincludes: disabling the first switch, enabling the second switch of theinternal buffer, and disabling the third and fourth switches of theinternal buffer to couple the buffered signal to the output pin.
 16. Themethod of claim 12, further comprising providing the driven signal to anexternal bipolar driver circuit coupled to the tuner when the tuner iscoupled to the AC-coupled load, and providing the buffered signal to atransmission line coupled to a processor when the tuner is coupled tothe DC-coupled load.
 17. A system comprising: an antenna to receive aradio frequency (RF) signal; a tuner coupled to the antenna, the tunerincluding: a mixer to mix the RF signal to a second frequency signal; adigitizer to digitize the second frequency signal; a digital signalprocessor (DSP) coupled to the digitizer to process the digitized secondfrequency signal and to demodulate the processed digitized secondfrequency signal when the RF signal includes an analog TV signal; adriver to output the analog TV signal to an output pin in a first mode;and a buffer coupled to the driver to receive the analog TV signal andto provide the analog TV signal to the output pin as a buffered signalin a second mode.
 18. The system of claim 17, further comprising aconfiguration register to store a control value.
 19. The system of claim18, wherein the control value is to enable the buffer in the second modewhen the tuner is coupled to a processor via a transmission line and todisable the buffer in the first mode when the tuner is coupled to theprocessor via an external driver.
 20. The system of claim 18, whereinthe buffer comprises: a converter to convert the analog TV signal to acurrent signal; a first current mirror to receive the current signal andto amplify the current signal to a first amplified current signal; and asecond current mirror coupled to the first current mirror to receive thefirst amplified current signal and to amplify the first amplifiedcurrent signal to the buffered signal, and to output the buffered signalto the output pin.